In an image-forming process of an electrophotographic system, an electrostatic latent image is formed on an inorganic photoreceptor formed of selenium, a selenium alloy, cadmium sulfate, amorphous silicon, etc. or an organic photoreceptor using a charge generation materials and a charge transport materials, developed with a toner, and transferred and fixed to a paper sheet or a plastic film to obtain a visible image.
Photoreceptors are classified into positively charged ones and negatively charged ones depending upon their structures. In the case where a printed portion is allowed to remain as an electrostatic latent image by light exposure, the latent image is developed with a toner charged with reverse polarity. In contrast, in the case where a printed portion is electrically discharged and subjected to reversal development, the printed portion is developed with a toner charged with the same polarity.
A toner contains a binder resin, a colorant and other additives. To impart desirable charging characteristics (charge rate, charge level, charge stability, etc.), temporal stability, environmental stability and the like, a charge control agent is generally added. Owing to the addition of the charge control agent, the characteristics of a toner are greatly improved.
Examples of a positive triboelectric charge control agent presently known in the art include a nigrosine dye, an azine dye, a copper phthalocyanine pigment, a quaternary ammonium salt and a polymer having a quaternary ammonium salt at a side chain. Examples of a negative triboelectric charge control agent presently known in the art include metal complexes of a monoazo dye, metal complexes of salicylic acid, naphthoic acid or dicarboxylic acid, a copper phthalocyanine pigment and a resin containing an acid component.
Furthermore, in the case of a color toner, market expansion of which is expected in the future, a pale-colored charge control agent having little effect on hue, desirably a colorless charge control agent, is indispensable. Examples of such a pale-colored or colorless charge control agent for use in a negatively charged toner include metal complex compounds of hydroxy benzoate derivatives (see, for example, Patent Literatures 1 to 3), metal salt compounds of aromatic dicarboxylic acids (see, for example, Patent Literature 4), metal complex compounds of anthranilic acid derivatives (see, for example, Patent Literatures 5 and 6), organic boron compounds (see, for example, Patent Literatures 7 and 8), biphenol compounds (see, for example, Patent Literature 9), calix[n]arene compounds (see, for example, Patent Literatures 10 to 15) and cyclic phenol sulfates (see, for example, Patent Literatures 16 to 18). Furthermore, examples thereof for use in a positively charged toner include quaternary ammonium salt compounds (see, for example, Patent Literatures 19 to 21).